Solid state image pickup device

ABSTRACT

A solid state image pickup device includes a sensor section having a plurality of light-receiving regions arranged such that each of the light-receiving regions converts an incident light into signal charges of an amount corresponding to the amount of light. A charge transfer register transfers the signal charges read out from each of the light-receiving regions of the sensor section. A charge discharging section discharges charges stored in each of the light-receiving regions of the sensor section under control of control means such that charges stored in each of the light-receiving regions are discharged to the charge discharging section within a predetermined period of time upon rising voltage of the power supply.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid state image pickup devicereferred to as a linear sensor (line sensor) or an area sensor, and moreparticular to a solid state image pickup device suitable for use as animage sensor for a bar-code reader for reading a bar code appended to amedium such as a commercial product.

2. Description of Related Art

FIG. 10 shows an example of a solid state image pickup device, forexample, a CCD linear sensor. The CCD linear sensor comprises a sensorrow 102 having a plurality of light-receiving regions 101 arranged inone row for converting an incident light into signal charges of anamount corresponding to the amount of the light and storing them; and acharge transfer register 104 comprising a CCD for transferring in onedirection the signal charges read out by way of a readout gate 103 fromeach of the light-receiving regions 101 of the sensor row 102.

The readout gate 103 reads out the signal charges stored in each of thelight-receiving regions 101 of the sensor row 102 simultaneously uponapplication of a readout pulse φROG. The charge transfer register 104transfers the signal charges using 2-phase driving by 2-phase transferclocks φH1 and φH2.

A charge/voltage conversion section 105, for example, of a floatingdiffusion constitution is formed at the final stage of the chargetransfer register 104 for converting the transferred signal charges intovoltage. The output voltage of the charge/voltage conversion section 105is led out by way of a buffer 106 from an output terminal 107 as a CCDoutput.

The CCD linear sensor having the above-mentioned constitution is used asan image sensor of a bar-code reader for reading a bar code appended,for example, on a medium such as a commercial product.

By the way, since the linear sensor is in a thermally balanced statebefore turning on a power supply and, the light-receiving regions 101are overflown with charges in the balanced state, pixel signals cannotbe read out normally owing to the presence of unnecessary initialcharges immediately after the turning on of the power supply voltage(rising of power supply). That is, the pixel signals cannot be read outnormally unless the unnecessary charges are discharged, so that start upof the device is delayed.

Since such unnecessary initial charges are increased as the aperture ofthe sensor section (sensor area) becomes greater, unnecessary charges inthe light-receiving regions 101 cannot be discharged during usualtransfer operation unless charges are read out and transferredrepeatedly for several to several tens of lines. Particularly, in a caseof a linear sensor applied to a bar-code reader, since the aperture sizeof 1 pixel is as large as 14 μm×200 μm and, therefore, the initialcharges in the light-receiving regions 101 accumulate in a great amountupon rising of the power supply voltage, so that the start up of thedevice till it operates normally after discharging the initial chargesis extremely delayed. By the way, the aperture size for one pixel of ausual linear sensor is smaller, for example, about 7 μm×7 μm or 14 μm×14μm as compared with a linear sensor used for the bar-code reader.

However, in a case of the bar-code reader, since the power supply iskept off during a stand-by state and reading has to be started instantlywhen a user turns on a power supply switch immediately before the use,so that it is necessary to rapidly read a bar code when the power supplyturns on.

In view of the above in the existent CCD linear sensor, as shown in FIG.11, it is adapted to rapidly discharge the initial charges by high speedtransfer of the signal charges by making the frequency for the φH1 andφH2 higher than in the usual transfer for a certain period of time afterturning on of the power supply.

However, in the linear sensor of the structure in which the initialcharges are discharged in a high speed transfer mode upon turning on ofthe power supply as described above, since two kinds of transfer clocksφH1 and φH2, that is, at high frequency for high speed transfer mode andat low frequency for usual transfer mode are necessary, a timinggenerator for generating such clocks is necessary complicating theconstitution and increasing the cost.

Further, since there is also a limit for increasing the frequency forthe transfer clocks φH1 and φH2 in the high speed transfer mode, thereis a limit for shortening the period of time to transfer to the normaloperation after turning on of the power supply.

SUMMARY OF THE INVENTION

The present invention has been accomplished in view of the foregoingproblems and an object thereof is to provide a solid state image pickupdevice capable of transferring to a normal operation in a short periodof time upon rising of the power supply voltage with a simpleconstitution, without providing a driving system with a function such ashigh speed transfer.

The foregoing object of the present invention can be attained inaccordance with the present invention by a solid state image pickupdevice comprising a sensor section having a plurality of light-receivingregions for converting an incident light into signal charges of anamount corresponding to the amount of the light and storing the same, acharge transfer register for transferring signal charges read out fromeach of the light-receiving regions in the sensor section, chargedischarging section for discharging the charges stored in each of thelight-receiving regions and a control means such that the charges storedin each of the light-receiving regions are discharged to the chargedischarging section within a certain period of time upon the rising ofthe power supply voltage.

In the solid state image pickup device of the foregoing constitution,each of the light-receiving regions of the sensor section is overflownwith unnecessary initial charges before turning on of the power supply.The initial charges are forcibly discharged under the control by thecontrol means to the charge discharging section upon rising power supplyvoltage.

In this instance, since the initial charges in each of thelight-receiving regions are discharged simultaneously to the chargedischarging section, it can rapidly transfer to a state capable ofcorrectly reading out the signal charges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a constitution of a first embodimentaccording to the present invention;

FIG. 2 is a timing chart for the first embodiment;

FIG. 3 is a view illustrating the constitution of a second embodimentaccording to the present invention;

FIG. 4 is a timing chart for the second embodiment;

FIG. 5 is a view illustrating the constitution of a third embodimentaccording to the present invention;

FIG. 6 is a timing chart for the third embodiment;

FIG. 7 is a circuit diagram illustrating an example of a circuitstructure of an inverter;

FIG. 8 is a characteristic view showing an input/output characteristicof the inverter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be made of preferred embodiments according to thepresent invention in detail with reference to the drawings.

FIG. 1 is a view illustrating the constitution of a first embodimentaccording to the present invention. In FIG. 1, light-receiving regions(pixels) comprising, for example, multiple photodiodes are arranged in arow (for example 2,000 pixels), for converting an incident light intosignal charges of an amount corresponding the amount of the light andstoring them to constitute a sensor row 2.

Signal charges stored in each of the light-receiving regions 1 of thesensor row 2 are read out simultaneously by way of a readout gate 3 to acharge transfer register 4. The readout gate 3 is supplied with areadout pulse φROG by way of a clock terminal 5.

On the other hand, the charge transfer register 4 is supplied with 2phase transfer clocks φH1 and φH2 through clock terminals 6 and 7. Thecharge transfer register 4 comprises a CCD and transfers the signalcharges read out from each of the light-receiving regions 1 in onedirection (rightward in the figure) under 2 phase driving by the 2-phasetransfer clocks φH1 and φH2.

A charge/voltage conversion section 8, for example, of a floatingdiffusion type is disposed at the final stage of the charge transferregister 4 for detecting the transferred signal charges and convertingthem into voltage. The output voltage from the charge/voltage conversionsection 8 is led out by way of a buffer 9 from an output terminal 10 asa CCD output. Thus, a CCD linear sensor, for example of 2,000 pixels isconstituted.

Further, in this embodiment, a charge discharging drain 12 is formed byway of the charge discharging gate 11 to the sensor row 2 on the sideopposite to the charge transfer register 4. The charge discharge gate 11is usually in a low potential state and functions as a potential barrierfor each of the light-receiving regions 1 of the sensor row 2.

The charge discharging gate 11 is supplied with a control voltage V1 fora predetermined period of time from a time constant circuit 14 as acontrol means upon rising voltage of a DC power supply (device powersupply) 13.

The time constant circuit 14 is formed on a substrate 15 identical withthat for the sensor row 2 or the charge transfer register 4, etc. (onchip), and it outputs the control voltage V1 for a certain period oftime in response to a power supply voltage Vdd applied from the DC powersupply through a power supply terminal 16.

Descriptions will then be made of the operation of the CCD linear sensoraccording to the first embodiment as described above with reference to atiming chart shown in FIG. 2.

In this CCD linear sensor, unnecessary charges are stored as initialcharges in each of the light-receiving regions 1 of the sensor row 2before turning on of the power supply. When the power supply is turnedon in this state, the control voltage V1 is outputted from the timeconstant circuit 14 for a predetermined period of time τ (for example,about 1 ms) in response to the power supply voltage Vdd applied throughthe power supply terminal 16.

The control voltage V1 is applied to the charge discharging gate 11.Then, the potential of the charge discharging gate 11 functioning thusfar as the potential barrier to each of the light-receiving regions 1 ofthe sensor row 2 becomes high by the application of the control voltageV1.

Thus, the initial charges stored in each of the light-receiving regions1 of the sensor row 2 are discharged by way of the charge discharginggate 11 to the charge discharging drain 12.

After elapse of the certain period of time τ from the turning on of thepower supply, the control voltage V1 disappears. Then, the potential ofthe charge discharging gate 11 becomes lowered and the gate 11 functionsagain as the potential barrier to each of the light-receiving regions 1of the sensor row 2.

Subsequently, signal charges corresponding to the image information tobe read (bar code information in a case of using an image sensor for thebar-code reader) are stored in each of the light-receiving regions 1 ofthe sensor row 2.

The signal charges stored in each of the light-receiving regions 1 ofthe sensor row 2 are simultaneously read out to the charge transferregister 4 under the application of the readout pulse φROG to thereadout gate 3 and then transferred successively when the chargetransfer register 4 undergoes 2-phase driving by the transfer clocks φH1and φH2.

Then, they are converted into voltage by the charge/voltage conversionsection 8 and read out by way of the buffer 9 from the output terminal10 as the CCD output.

As described above, since the charge discharging gate 11 and the chargedischarging drain 12 are formed in the sensor row 2 on the side oppositethe charge transfer register 4, and the control voltage V1 is applied tothe charge discharging gate 11 from the time constant circuit 14 for acertain period of time upon rising power supply voltage, the unnecessaryinitial charge stored in each of the light-receiving regions 1 can bedischarged rapidly to the charge discharging drain 12 before turning onof the power supply.

This enables rapid transfer to the normal operation upon rising voltageof the power supply by a simple structure.

When the initial charges are discharged upon rising power supplyvoltage, it is not always necessary to discharge all the initial chargesand, if the initial charge can be partially discharged, a correspondingeffect can be obtained.

Further, when the readout pulse φROG is applied to the readout gate 3and the transfer clocks φH1 and φH2 are applied to the charge transferregister 4 to conduct usual readout/transfer operation in parallel withthe discharging operation of the initial charges, the initial chargescan be discharged not only from the charge discharging gate 3 but alsofrom the readout gate 11.

FIG. 3 is a view illustrating the constitution of a second embodimentaccording to the present invention in which sections corresponding tothose in FIG. 1 carry the same reference numerals.

In this second embodiment, a time constant circuit 14 is formed outsidea substrate 15 and a control voltage V1 outputted from the time constantcircuit 14 in response to rising DC voltage Vdd is taken by way of acontrol terminal 17 into the substrate 15 and, further, applied by wayof a buffer 18 for processing such as waveform shaping to a chargedischarging gate 11. Other constitutions are the same as those in thefirst embodiment.

Accordingly, the operation of the second embodiment having the foregoingconstitution is quite identical with that in the first embodiment andcan provide the same effect. FIG. 4 is a timing chart for the secondembodiment.

In the second embodiment, the control voltage V1 taken through thecontrol terminal 17 into the substrate 15 is applied by way of thebuffer 18 to the charge discharging gate 11. If processing such aswaveform shaping is not necessary, it may be applied directly to thecharge discharging gate 11 not by way of the buffer 18.

FIG. 5 is a view illustrating the constitution of a third embodimentaccording to the present invention in which sections corresponding tothose in FIG. 3 carry the same reference numerals.

In this third example, a CR time constant 19 comprising a capacitor Cconnected between a control terminal 17 and the ground and a resistor Rconnected at one end to the control terminal 17 and applied at the otherend with a power supply voltage Vdd is used, instead of the timeconstant circuit 14 in the second embodiment.

Further, the output voltage V0 of the CR time constant circuit 19 istaken by way of the control terminal 17 into a substrate 15 and thenapplied by way of an inverter 20 to the charge discharge gate 11. Inthis case, the power supply voltage rising characteristic of the CR timeconstant circuit 19 is represented as:

    V(t)=Vdd·(1-exp) (-t/CR))

The inverter 20 is disposed for waveform shaping.

Then, description will be made of the operation of the CCD linear sensoraccording to the third embodiment having the foregoing constitution withreference to a timing chart shown in FIG. 6.

At first, when the power supply is turned on, the output voltage V0 ofthe CR time constant circuit 19 gradually increases in response to apower supply voltage rising characteristic represented by theequation 1. Further, the control voltage V1 as the output voltage of theinverter 20 receiving the output voltage V0 at the input risessubstantially upon turning on of the power supply.

When the control voltage V1 is applied to the charge discharging gate11, the potential of the charge discharging gate 11 that has functionedso far as the potential barrier to each of the light-receiving regions 1of the sensor row 2 is increased by the application of the controlvoltage V1.

Thus, the initial charges stored in each of the light-receiving regions1 of the sensor row 2 are discharged by way of the charge discharginggate 11 to the charge discharging drain 12.

When the output voltage V0 of the CR time constant circuit 19 rises andreaches a threshold level of the inverter 20 with lapse of a certainperiod of time τ from the instance the power supply turns on, the outputof the inverter 20 is inverted to eliminate the control voltage V1.Then, the potential of the charge discharging gate 11 is lowered and itfunctions again as the potential barrier to each of the light-receivingregions 1 of the sensor row 2. Subsequently, signal chargescorresponding to the bar code information are stored in each of thelight-receiving regions 1 of the sensor row 2.

Usual readout/transfer of the signal charges from each of thelight-receiving regions 1 of the sensor row 2 are conducted in the samemanner as in the first embodiment.

In this third embodiment, the CR time constant circuit 19 is disposedoutside the substrate 15 but it may be disposed also in an on-chipmanner like that in the first embodiment.

On the contrary, the inverter 20 is disposed on-chip but it may bedisposed outside the substrate 15. However, if it is disposed out of thesubstrate 15, an additional power supply for the inverter 20 is requiredand, therefore, on-chip arrangement is preferred.

FIG. 7 is a circuit diagram illustrating an example of a circuitstructure for the inverter 20. As shown in the figure, the inverter 20has a CMOS structure comprising a P-channel MOS transistor M1 and anN-channel MOS transistor M2 connected in series between a power supplyVdd and the ground.

Then, they are wired such that an input voltage Vin is applied to acommon junction for each gate of both MOS transistors M1 and M2, and anoutput voltage Vout is led out from the common junction for each of thedrains.

In a case of using the inverter of this kind as a usual logic circuit,it is typical to constitute the inverter as a so-called center typeinverter showing an input/output characteristic as shown by the brokenline in FIG. 8 in which a threshold level is set to about 1/2 of thepower supply voltage Vdd (Vdd/2). On the other hand, in this embodiment,it is constituted as a so-called high threshold type inverter showing aninput/output characteristic as shown by the solid line in FIG. 8 inwhich the threshold level is set to higher than 1/2 of the power supplyvoltage Vdd.

Since the inverter 20 is constituted as the high threshold typeinverter, a predetermined period of time τ can be set larger as comparedwith a case of the center type inverter constitution without increasingvalues for the capacitor C and the resistor R of the CR time constantcircuit 19 as can be seen from the waveform chart in FIG. 8.

In other words, assuming an identical certain period of time, the valuesfor the capacitor C and the resistor R of the CR time constant circuit19 can be made smaller than in the case of the center type inverter, sothat inexpensive parts can be used to provide an economical advantage.As an example, an expensive electrolytic capacitor or tantalum capacitorhas to be used in a case of a large capacitance value but an inexpensiveceramic capacitor may suffice in a case of a smaller capacitance value.

In a case of constituting the inverter 20 as the high threshold typeconstitution, the P-channel MOS transistor M1 and the N-channel MOStransistor M2 may be set such that the mutual conductance g_(m1) andg_(m2) may be set as g_(m1) g_(m2), specifically, g_(m2) /g_(m1) isabout from 1/2 to 1/10.

Further, the g_(m2) /g_(m1) ratio may be changed by, for example,changing w (channel width)/L (channel length) that is a parameterrepresenting each of the gains for the P-channel MOS transistor M1 andthe N-channel MOS transistor M2.

In this embodiment, the CMOS structure is used for the inverter 20 butthis is not limited thereto and an inverter of other types such as E/Dconstitution or E/E constitution using N-channel MOS may be used and,further, a Schmitt trigger inverter resistant to noises may be used.

Also in a case of using such inverters, the same effect as describedabove can be obtained by constituting them as the high threshold typeinverter.

In each of the embodiments, the charge discharging gate 11 and thecharge discharging drain 12 are additionally disposed for dischargingunnecessary initial charges. However, if it is applied to a linearsensor having a so-called electronic shutter function, since the shuttergate and the shutter drain can be used also as the charge discharginggate 11 and the charge discharging drain 12, the aimed object can beattained by a simple structure of only adding the time constant circuit.

Further, in each of the embodiments, a so-called lateral structure isadopted in which the initial charges in each of the light-receivingregions 1 of the sensor row 2 are discharged by way of the chargedischarging gate 11 to the charge discharging drain 12. However, it maybe a so-called vertical constitution of discharging the initial chargesto the substrate 15 by applying the control voltage V1 to the substrate15.

Further, the present invention is not always restricted to the CCDlinear sensor application but it is applicable to all solid state imagepickup devices including area sensors not restricted only to CCD.

As has been explained above, according to the present invention, sincethe charge discharging section for discharging the charges stored ineach of the light-receiving regions of the sensor section is disposedand unnecessary initial charges are forcibly discharged to the chargedischarging section upon rising voltage of the device power supply, itcan rapidly start to the normal operation upon rising power supplyvoltage with a simple structure without providing the driving systemwith a function such as high speed transfer.

What is claimed is:
 1. A solid state image pickup device receiving apower signal from a power supply, comprising:a sensor section receivingthe same said power signal and having a plurality of light-receivingregions, each of the light-receiving regions converting an incidentlight into signal charges of an amount corresponding to the amount ofthe incident light; a charge transfer register receiving the same saidpower signal, for transferring the signal charges read out from each ofthe light-receiving regions of the sensor section; a charge dischargingsection receiving the same said power signal, for discharging initialcharges stored in each of the light-receiving regions of the sensorsection; and a time constant circuit, receiving the same said powersignal, for generating a control signal of a predetermined duration inresponse to the same said power signal generated by the power supplybeing turned on, the time constant circuit supplying the control signalto the charge discharging section such that the initial charges storedin each of the light-receiving regions are discharged to the chargedischarging section during the predetermined signal duration.
 2. A solidstate image pickup device as defined in claim 1, wherein the chargedischarging section comprises a drain section formed in the sensorsection on the side opposite the charge transfer register, and a gatesection controlled by the time constant circuit for discharging theinitial charges stored in each of the light-receiving regions of thedrain section.
 3. A solid state image pickup device as defined in claim2, wherein the time constant circuit comprises a CR time constantcircuit the output of which increases as a function of a predeterminedtime constant in response to the rising of the power supply voltage, andan inverter for inverting the output of the CR time constant circuitabove a threshold level for setting a predetermined period of time andapplying the inverted output to the gate section.
 4. A solid state imagepickup device as defined in claim 3, wherein the threshold level is setto a value higher than 1/2 of the power supply voltage.
 5. A solid stateimage pickup device as defined in claim 1, wherein the light-receivingregions are arranged in a one dimensional line.
 6. A solid state imagepickup device as defined in claim 1, wherein the incident light is areflection light of a light irradiated from a light source to a bar codeappended to a medium.